This is a U.S. original application which claims priority on French patent application No. 0202860 filed Mar. 7, 2002.
The present invention is in the photographic field, and is used to measure the sensitometric characteristics of a photographic element. The present invention relates more particularly to the production of a sensitometric control patch with an exposure gradient that is placed on a photographic element and enables the measurement of the sensitometric data of the photographic element.
In the photographic field, it is beneficial to control the entire chain of use and processing of a photographic film. In this field, the use of sensitometric control patches is known to those skilled in the art. The sensitometric control patches of a photographic element (film or photo paper) comprise a set of sensitometric data that enable the photographic element to be calibrated against preset nominal values. From each of these control patches, the sensitometric curve specific to the photographic element is thus reproduced. The photographic element is, for example, a film strip or a strip of photographic paper.
The response of a photographic film to light exposure varies according to, for example, its manufacturing process, the period and conditions of its storage, etc. The photographic processing chain can include xe2x80x9canalogxe2x80x9d operations of chemical development type, or digital operations. Productivity constraints require fast adjustment of apparatus setting parameters, like for example, a picture-taking camera or a digitizer (scanner), according to the sensitometric response specific to a film strip used, for example, in the recording camera. So it is particularly useful to know the sensitometric characteristics (optical densities) specific to the film strip, and to integrate them on-line in the apparatus used (camera, digitizer), to adapt the settings of the apparatus at the same time as the film is used (exposure, digitization). A targeted objective is to reproduce for the final customer images of an initial scene, having good reproduction quality without losing time. Therefore, and in relation to the medium or photographic element used, one means known to those skilled in the art is to reconstitute the sensitometric curve specific to this photographic element, using sensitometric control patches specific to the photographic element.
U.S. Pat. No. 6,284,445 describes a photographic element and a method that enables the area occupied by a plurality of sensitometric control patches to be optimized on the photographic element. The method also enables measurement of the sensitometric characteristics of the photographic element to be controlled, using a particular layout of the sensitometric control patches on the element.
U.S. Pat. No. 3,718,074 describes a camera that includes a device that enables the sensitometric control patches incorporated in the film strip used in the camera to be exposed. The means described in this patent are cumbersome.
U.S. Pat. No. 5,563,717 describes a particular method to produce a plurality of sensitometric control patches of constant width and different exposure levels, on a photographic element, by using a minimum surface area of the photographic element. The photographic element is, for example, a 35 mm photo film. U.S. Pat. No. 5,563,717 enables, for example, the dimensions of the means as described in U.S. Pat. No. 3,718,074 to be optimized.
U.S. Pat. No. 5,667,944 describes a method that enables corrected photographic images to be obtained taking into account the actual optical density values of the negative film strip used to record the latent image originally recorded. The described method comprises forming at least two reference sensitometric control patches having different exposure levels on the photographic film strip.
Patent Application DE 26,32,715 describes a correction method for color photographic films that uses three sensitometric control patches of different exposure levels on a film. The sensitometric control patches are placed between the perforations and outside the individual image locations on the film. The differences between the actual values of the sensitometric characteristics read of these sensitometric control patches and their nominal values enable color and image errors to be corrected. Document DE 26,32,715 also describes a camera including an exposure system for sensitometric control patches that enables, at each exposure of an individual image, the automatic exposure of a sensitometric control patch, so that the individual image can be corrected later during copying or digital conversion.
Contrary to the image quality requirements, the technical constraints of speed (several tens of images a second) and dimensions (reduced dimensions and weight of equipment), entail using as few sensitometric control patches as possible. These constraints mean that the exposure, for example, of a sensitometric control patch for each image recorded in a camera, at a rate of 150 images a second, requires perfectly controlling the energy and the exposure time of the control patch that will be used for reference. The accuracy of the corresponding sensitometric measurements will be all the more difficult to obtain, because of the variability generated by the camera""s electronic and/or mechanical elements, and the repetition of the exposures. Furthermore, even using advanced recording means, reading and saving the data for each of the sensitometric control patches requires a time incompatible with, for example, the running speed of the photographic elements.
The present invention is based on obtaining the optical density curve of a photographic element as a function of the illumination, by the deconvolution of a unique sensitogram whose illumination function has been determined. Deconvolution relates to determining, point by point, the characteristic function of the sensitometric curve of the photographic element after development, and separating the characteristic function from the illumination function of the photographic element.
The present invention relates more particularly to an exposure system of a photographic element comprising a light energy source placed along the photographic element. The light energy source is servo connected by an electrical link to a management module of the exposure parameters, and the exposure system enables a relative movement between the light energy source and the photographic element. The exposure system enables a light pulse to be produced whose respective digital values of the instant of emission, of the intensity, and the duration of the emission are automatically computed according to the nominal sensitometric characteristics of the photographic element. The values are automatically modified in time according to the instantaneous speed of relative movement of the light energy source in relation to the photographic element.
The present invention further relates to the use of the exposure system presented below, to produce a reference sensitometric control patch on the photographic film strip placed in a recording or projection camera.
Compared with the means described in the prior art, the present invention has the advantage of only using a single sensitometric control patch per photographic element. This sensitometric control patch has the particularity of having non-uniform exposure levels. Compared with the prior art, the present invention has the advantage, by meeting the above-mentioned speed and dimensions constraints, of only requiring a single reference sensitometric control patch per photographic element to be calibrated. The surface area and the time of the exposure sequence of the sensitometric control patch are thus optimized. The advantage provided by the invention is thus significant, given the above-mentioned constraints of time, measurement accuracy, and dimensions.
Other characteristics will appear on reading the following description, with reference to the drawings of the various figures.